Method and apparatus for forming an article of high purity metal oxide

ABSTRACT

A torch flame resulting from the combustion of gaseous silicon tetrachloride and a mixture of hydrogen and oxygen is directed upon a mandrel in order to form a high purity silica article thereon. The torch includes a nozzle aperture for providing an output jet stream of silicon tetrachloride entrained in a carrier gas. A sheath of oxygen containing gases is streamed about the jet stream of silicon tetrachloride in order to prevent reaction immediately adjacent the face of the torch nozzle with a stream of combustible gas provided about the sheath of gas. When the torch is ignited, the resulting flame is directly applied to the mandrel wherein a layer of high purity silica is deposited thereon due to the vapor phase hydrolysis of the silicon tetrachloride.

llnited States Patent 72] Inventor Michael A. Carrel! 3,364,970 1/1968Dombruch 239/424.5X Plano, Tex. 3,387,784 6/1968 Ward, Jr 239/1323 [21]Appl.N0. 744,188 3,411,717 11/1968 Flynn 239/132.3 Filed July 11, 32FOREIGN PATENTS [45] Patented Feb. 23, l l Assignee Texas InstrumentsIncorporated 1,249,283 11/1960 France 239/132.3

Dallas, Tex. Primary Examiner-Lloyd L. King AttorneysSamuel M. Mims,Jr., James 0. Dixon, Andrew M.

Hassell, Harold Levine, Melvin Sharp, Gerald B. Epstein, [54] METHOD ANDAPPARATUS FOR FORMING AN John E. Vandigriff and Richard, Harris andHubbard ARTICLE OF HIGH PURITY METAL OXIDE 12Clai 6D a in F s.

r w E lg ABSTRACT: A torch flame resulting from the combustion of [52]US. Cl. 239/422, gaseous ili t trachloride and a mixture of hydrogen and239/132-3 239/424-5 oxygen is directed upon a mandrel in order to form ahigh pu- [51] Int. Cl. F 23d 11/16 m silica article thereon The torchincludes a nozzle aperture [50] Field of Search 239/1323, for providingan output jet stream of Silicon tetrachloride em 422 trained in acarrier gas. A sheath of oxygen containing gases is 56] Ref Cit dstreamed about the jet stream of silicon tetrachloride in order erencese to prevent reaction immediately adjacent the face of the torch UNITEDSTATES PATENTS nozzle with a stream of combustible gas provided aboutthe 2,719,581 10/1955 Greathead 239/132.3 sheath of gas. When the torchis ignited, the resulting flame is 2,827,112 3/1958 lnskeep 239/ 132.3directly applied to the mandrel wherein a layer of high purity 3,224,6799 K 6! 23 132-3 silica is deposited thereon due to the vapor phasehydrolysis of 3,339,616 9/1967 Ward, 11'. et al. 239/1 32.3 the silicontetrachloride.

v P I I 6 I I l l PATE NTEDFEBZSISYI 3.565.346

' SHEET 1 BF 2 FIG. 4

I INVENTOR MICHAEL A. CARRELL PATENTEDFEB23197| INVENTOR MICHAEL A.CARRELL MIETI'IGD AND APPARATUS FOR FORMWG AN ARTICLE h HIGH PURI'IYMETAL 015 This invention relates to the production of metal oxide by thedecomposition of volatile metal chlorides, and more particularly to thedeposition of metal oxide upon a substrate by the vapor phase hydrolysisby a flame of volatile anhydrous chlorides of metallic elements fromgroups Ill and IV of the periodic system, as for example, silicontetrachloride, titanium tetrachloride, aluminum tetrachloride and tintetrachloride.

In the formulation of certain semiconductor devices, monocrystallinesilicon is pulled" from a melt of silicon. Due to the extremely hightemperatures required to provide a melt of silicon, it has been foundthat appreciable quantities of the material from which the crucible isconstructed is lost within the melt. l-Ience, any impurities in thecrucible enter into the pulled monocrystalline silicon. It has thus beenfound advantageous to construct the silicon melt crucible from very puresilica. It is also extremely important to provide a uniform pull" fromthe silicon melt, and it is thus necessary that the crucible have veryuniform sidewalls and a symmetrical configuration in order to preventsudden changes in the level of the melt during pulling due toirregularities in the crucible configuration.

Various techniques have been heretofore devised for making articles fromsilica. For instance, U.S. Pat. No. 3,117,838, issued Jan. 14, 1964,discloses the utilization of a flaming torch for oxidizing a gas mixtureincluding silane and a reactive gas to form molten silica and directingthe molten silica onto a carbon form to grow a body of transparentsilica. Further, US. Pat. No. 2,272,342, issued Feb. 10, 1942, disclosesthe production of a transparent article of silica by vaporizing siliconfluoride or silicon tetrachloride and decomposing the vapor in a flame.The flame is then impinged on a refractory core to deposit a layer ofsilica, whereupon the silica is vitrified to a transparent article bythe application of high temperature. However, such previously developedtechniques have not been completely satisfactory with respect to formingan extremely pure silica article having the desired uniform andsymmetrical configuration, and having the necessary strength for use asa melt crucible.

It has been known that one can produce finely divided metal oxide fromvolatile metallic chloride by supplying a stream of the vaporizedvolatile metal chloride through a nozzle into a reactor and supplyingoxygen and other gases to support combustion. The gas streams are thenignited in the reactor to decompose and oxidize the volatile metalchloride to form finely divided oxide which is withdrawn from the bottomof the reactor. In order to prevent obstruction of the nozzle due tobuildup of crystals of the volatile metal chloride, it has heretoforebeen known to provide an alternate layer of relatively inert gas betweenthe combustible supply of gas and the vaporized volatile metallicchloride. Examples of such systems are disclosed in U.S. Pat. No.2,240,343, issued Apr. 29, 1941; U.S. Pat. No. 2,394,633, issued Feb.12, 1946; and US. Pat. No. 2,823,982,issued Feb. 18, 1958.

In addition to the fact that such previously known burners havegenerally been useful only in a reaction chamber to form loose powderedoxide, such burners have often provided lower than desired efficiency inthe production of oxide. One reason for this low efliciency is thoughtto be the fact that relatively large amounts of vaporized metallicchloride have been provided at high mass flow rates through large burneropenings, thereby resulting in uneven contact of the different gasmixtures in certain regions in the flame. Further, such techniques havegenerally utilized oxidation processes rather than the hydrolysis of thevolatile metal chloride. For these and other reasons, such burners havenot been generally useful outside such reaction chambers for the directapplication of silica upon mandrels where high deposition efficiency andeven application of silica is required.

In accordance with the present invention, a volatile metallic chlorideis vaporized and entrained in a carrier gas and then streamed from a jetnozzle. A stream of combustible gas is formed symmetrically about thejet stream, with interaction between the two streams being prevented ina preselected region adjacent the jet nozzle. The streams are ignited ata zone of interaction to form a flame which is directed upon a mandrelto form an article of high purity oxide directly thereupon.

In a more specific aspect of the invention, a torch is provided with acentral passage therethrough for receiving gaseous silicon tetrachlorideentrained in carrier gas. A nozzle aperture is defined in one end of thepassage to provide an output jet stream of gaseous silicon tetrachlorideentrained in the carrier gas. A sheath chamber defined in the torchineludes an inlet for receiving an oxygen-containing gas and has anannular opening concentrically disposed about the nozzle aperture toprovide a circular stream of oxygen-containing gas around the jetstream. A mixing chamber is disposed about the sheath chamber within thetorch body and includes inlets for hydrogen and oxygen-containing gases.A plurality of output openings are disposed symmetrically around theannular sheath opening to provide streams of gaseous combustible mixtureto allow the torch to be ignited and directed upon a mandrel for thedeposition of high purity silica thereon.

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference may now be made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates somewhat diagrammatically the formation of a silicacrucible upon a rotating mandrel with the present torch;

FIG. 2 illustrates an embodiment of the present torch;

FIG. 3 illustrates an end view of the nozzle of the torch shown in FIG.2;

FIG. 4 illustrates a sectional side view of another embodiment of atorch according to the invention;

FIG. 5 is an end view illustrating the nozzles of the torch shown inFIG. 4; and

FIG. 6 is a sectional view taken generally along the section lines 6-6in FIG. 4.

Referring to FIG. I, the torch designated generally by the numeral 10provides a flame for providing vapor phase hydrolysis of a gaseousvolatile metal chloride to produce a metal oxide which is deposited upona rotating mandrel 12. In the preferred embodiment of the invention,silicon tetrachloride is decomposed to form silicon dioxide according tothe following equation:

The flame produced from the torch 10 is shown diagrammatically ascomprising a very hot central portion 14 wherein a vaporized silicontetrachloride stream 16 reacts with a concentric stream of combustiblegas 18 including a mixture of hydrogen and oxygen. The streams 16 and 18interact at some distance below the nozzle of the torch 10 in a reactionzone which is spaced a short distance from the nozzle of the torch 10.This spacing of the reaction zone from the torch nozzle is due to theprovision of a stream of sheath gas 20 around the stream of silicontetrachloride 16, as will be subsequently described in greater detail.

The mandrel 112 is preferably constructed from graphite and may includea suitable hard metal coating over the surface thereof. The mandrel 12is rotated in the direction of the arrow 22 and is translated in adirection of the arrow 24 while the torch 10 remains stationary. Asilica article is deposited upon the mandrel as it translates past thestationary torch 10. The rate of translation of the mandrel 12 isdependent upon the desired thickness of the silica article and uponparameters of the torch flame, such as flow rates of the various gasesand the like. After the silica article has been deposited to the desiredthickness upon the mandrel 12, the article is removed and treated withvarious desired processing steps to provide a final product. It has beenfound that the articles have sufficient green strength" to allow removalfrom the mandrel due to a slight degree of sintering of the silicondioxide particles during deposition. In some instances, a separateconventional torch flame may be impinged on the article after depositionthereof to insure that sufficient green strength is provided. Thismethod provides excellent silica crucibles with smooth wall surfaces andsymmetrical configurations fur use in drawing semiconductormonocrystalline pulls.

FIG. 2 illustrates in greater detail the preferred embodiment of thepresent torch 10. A stainless steel tube 30 extends through the lengthof the torch to provide a passage for vaporized silicon tetrachlorideentrained in a carrier gas. A T- connection 32 is connected about thetube and is sealed to the tube 30 by member 34. A member 36 fits over astainless steel tube 38 to provide a seal between the end of tube 38 andthe tube 30. An annular sheath chamber 40 is thus defined between thetube 30 and tube 38. An inlet portion 42 of the T- connection 32 isconnected to a source of sheath gas, in this instance oxygen-containinggas, so that the sheath gas is passed into the sheath chamber 40. Oneactual embodiment of the invention utilized a one-fourth inch diameterstainless steel tube 30 and a /;-inch diameter'stainless steel tube 38.

An annular spacer member 44 provides a fluid seal between the tube 38and one end of a housing 46. Housing 46 receives a tube 48 which definesan annular mixing chamber 50 about tube 58. An inlet fitting 52 isconnected to a source of hydrogen while an inlet fitting 54 is connectedto a source of oxygen. Housing 46 includes a large diameter portion 56which defines an annular cooling chamber about the tube 48. An inletfitting 58 is connected to a source of cooling fluid, such as coolwater, and an outlet fitting 60 allows exhausting of the cooling fluid.In one practical embodiment of the torch 10, the smaller diameterportion of the housing 46 was constructed of stainless steel, while thelarger diameter portion 56 of the housing was constructed of brass toprevent corrosion thereof by the cooling fluid. In this embodiment ofthe torch, tube 48 was constructed from stainless steel and was providedwith a diameter of 1 /8 inch. The entire length of the torch 10 in thisembodiment thereof was approximately 9 inches.

The nozzle 62 is fitted over the end of the torch 10. As shown in FIGS.2 and 3, the nozzle 62 comprises a circular member with bent flanges 64for fluid-tight connection to the torch. A central nozzle aperture 66 isprovided in the end of tube 30. The end of tube 30 is received by anopening in nozzle 62 to define an annular sheath opening 68 which isconcentrically disposed relative to the aperture 66 and has asubstantially greater area than the aperture 66. Nozzle aperture 66 hasa substantially smaller area than the interior area of the tube 30, suchthat a relatively high velocity jet of gas is provided from the nozzleaperture 66. Gas flowing from the concentric sheath opening 68substantially envelopes the jet of gas from the nozzle aperture 66 toprevent immediate reaction thereof, thereby reducing crystallineaccumulation of the face of the nozzle member 62 and preventingobstruction thereof.

A plurality of apertures 70 are symmetrically disposed around the nozzleaperture 66 in a cylindrical configuration and communicate with themixing chamber 50. A combustible gas contained in the mixing chamber 50flows out of the apertures 70 and reacts with the gas flowing from theaperture 66. In a preferred embodiment of the torch, eight apertures 70were provided, each having substantially the same area sa the nozzleaperture 66. In this practical embodiment of the torch, diameters of.063 inch were found advantageous for aperture 66 and apertures 70. Fordifferent operating conditions, greater or larger numbers of aperture 70may be used.

Oxygen is supplied via conduit 72 to the inlet of three flowmeters 74,76 and 78. A suitable source supplies hydrogen through a conduit 80 to aflowmeter 82. The oxygen and hydrogen are dried prior to entering theflowmeters. Suitable valves are provided on the output of each of theflowmeters in order to allow regulation of the rate of flow of thehydrogen and oxygen to the e system. Oxygen is fed via a conduit 84 to abubbler system designated generally by the numeral 86. Bubbler system 86comprises a container filled with liquid silicon tetrachloride. Adiffusing element 88 bubbles the oxygen upwardly through the silicontetrachloride, thereby entraining vapors of the silicon tetrachloride inthe oxygen and passing outwardly through the conduit 90.

Conduit 90 is connected to the tube 30 to provide a metered stream ofvaporized silicon tetrachloride entrained in the oxygen carrier gas.Oxygen is also supplied through the flowmeter 74 via a conduit 92 to theinlet fitting 42 of the T-connection member 32. Oxygen thus .flows intothe sheath chamber 40 and out the annular aperture 68 in the mannerpreviously described. Oxygen is further supplied via conduit 94 to theinlet fitting 54 for passage into the mixing chamber 50. Hydrogen issupplied through the flowmeter 82 via a conduit 96 into the inletfitting 52 for mixing with the oxygen inside the mixing chamber 50. Amixture of combustible gas is then fed outwardly through the aperture 70in the manner described. Because the hydrogen and oxygen are mixedwithin the small chamber 50 within the torch, flashback in the presenttorch is limited to the interior of the torch itself.

FIGS. 4-6 illustrate another embodiment of the invention where aplurality of flames are provided. A metal block 10 has defined therein agenerally rectangular chamber 102 including an inlet 104 for theadmission of silicon tetrachloride vapors entrained in a suitablecarrier gas.

Four tubular passages 106-112 extend from the chamber 102 to theopposite face of the body 100. Relatively small nozzle apertures ll4-120respectively communicate with the passages to provide jet streams of thereactant gases in a similar manner to that previously described. Achamber 122 is formed in the general central region of the block andincludes an inlet 124 for a sheath gas such as oxygen. The chamber 122includes four sheath passageways 126- 132 for passing theoxygen-containing gas outwardly through annular sheath gas openings 134-140.

A pair of chambers 142 and 143 are defined within the body 100 forrespectively receiving a mixture of oxygen and hydrogen. The mixture ofoxygen and hydrogen is then flowed outwardly through a plurality ofapertures 144 and 145 for reaction with the gaseous silicontetrachloride. When ignited, the torch provides four flames to providean even application of silicon dioxide upon a mandrel in the mannerpreviously described. Water cooled pipes may be disposed on the sides ofthe body in order to provide cooling thereof. The sheath of oxygenflowing from the annular apertures 134-140 prevent accumulation ofcrystals on the face of the nozzles and therefore prevent obstruction ofthe torch during usage thereof.

The following examples will further explain the present torches and theuses thereof, but should not be held to limit the true scope of thisinvention.

EXAMPLE 1 A torch constructed in accordance with FIGS. 2 and 3 wasconnected to a gas supply system similar to that shown in FIG. 2 andignited. The diameter of the center nozzle aperture of the torch was.049 inch. A nonrotating mandrel constructed of graphite was disposed 4inches from the nozzle of the torch and the flame issuing from the torchwas impinged upon the graphite mandrel for 20 minutes. The temperatureof the flame approximately one-fourth inch from the mandrel was in therange of 1,500 C. One liter per minute of oxygen and l .56 liters perminute of silicon tetrachloride entrained in the oxygen was provided tothe torch. To provide this flow of gas, the bubbler shown in FIG. 2 wasmaintained at a temperature of approximately 50" C. and at a pressure of5 pounds per square inch. The percentage of silicon tetrachloride in theoxygen-carrier gas in this instance was 60.9 percent. One liter perminute of oxygen was fed into the sheath chamber of the torch to providean output of sheath gas. 5.2 liters per minute of oxygen and 30 litersper minute of hydrogen were supplied to the mixing chamber of the torchto provide the combustible mixture to the flame. The resulting jet ofgaseous silicon tetrachloride occurring at the center aperture of thetorch was approximately 6.9 X 10 feet per minute. After 20 minutes ofdeposition, the actual deposition of silicon dioxide on the gra- EXAMPLE2 In use of the present torch, it was found that a percentage of silicontetrachloride concentration in the carrier gas below about 75 percentprovided excellent efficiency of deposition. 1

Optimum efficiency was usually obtained at percentages of silicontetrachloride below 60 percent of silicon tetrachloride in the carriergas stream. For instance, a 20 minute deposition was provided in themanner described in example 1, with the exception that only .75 litersper minute of silicon tetrachloride was entrained in 1 liter per minuteof oxygen.

' This was accomplished by maintaining a temperature of 396 C. upon thebubbler. 43 percent of silicon tetrachloride was entrained in thebubbler gas in this instance to provide a velocity at the bubbler nozzleaperture of 4.7 X 3 feet per minute. In this example, an actualdeposition of 21.3 grams of silicon dioxide was deposited to provide adeposition efficiency of 57 percent.

EXAMPLE 3 It was found that for a particular construction of the torchand with a particular combination of gas flow rates, an optimum distancebetween the torch nozzle and the mandrel existed which providedincreased deposition rates. For instance, an experiment was runutilizing essentially the same flow rates and other parameters ofexample 2, but with the distance between the torch nozzle and themandrel being 5 inches. In this example, only 15.9 actual grams ofsilicon dioxide were deposited to provide a deposition efficiency of 43percent. For the particular torch used, it was found that a mandreldistance in the range of 4 inches was desirable.

EXAMPLE 4 Good results were also obtained with use of the present torchby the utilization of a larger bubbler gas noule aperture at higher flowrates. For instance, a minute run was conducted utilizing a nozzlehaving a nozzle aperture of a diameter of .063 inches and spaced 4inches from a mandrel. A supply of oxygen at a rate of 2 liters perminute was provided to the bubbler which was maintained at 49 C. at apressure of 5 pounds per inch. Vaporized silicon tetrachloride wasentrained in the carrier gas at a rate of 2.88 liters per minute toprovide a percentage of silicontetrachloride in the entrained gas of 59percent. A supply of oxygen at a rate of 1 liter per minute was suppliedto the sheath chamber of the torch. A supply of oxygen at a rate of 5 .2liters per minute was supplied to the mixing chamber of the torch alongwith a supply of hydrogen at a rate of 30 liters per minute to providethe mixture of combustible gases. An output velocity at the nozzleaperture of 8 X 10 feet per minute was provided. A deposition of 68grams of silicon dioxide was provided by the 20 minute run to provide adeposition efficiency of 48 percent.

The present invention thus provides a unique technique for forming highpurity articles on mandrels by the direct application of oxides due tovapor phase hydrolysis by flame. The term oxygen-containing gases asused in the specification is meant to include pure oxygen. While thespecification has been disclosed specifically with respect to thedeposition of silicon dioxide by the decomposition of silicontetrachloride, it will be understood that other volatile anhydrouschlorides of metallic elements from groups Ill and IV of the periodicsystem, such as for example, titanium tetrachloride, aluminumtetrachloride and the like could additionally be advantageouslydecomposed with the present technique.

Although specific embodiments of the present invention have beendescribed, it will be understood that various modifications and changeswill be suggested to one skilled in the art, and it is intended toencompass such modifications and changes which fall within the truescope of the invention as defined in the appended claims.

Iclaim: N

1. A torch for decomposing a volatile metallic element by hydrolysis todirectly form an oxide article on a surface comprising;

a. a torch housing including a passage terminating in a nozzle aperturefor providing an output jet stream of said volatile metallic elemententrained in a carrier gas;

b. means defining a first chamber disposed adjacent said passage withinsaid torch housing for receiving a supply of a combustible gas andincluding nozzle openings symmetrically disposed to said nozzle aperturefor providing a stream of combustible gas about said jet stream; and

c. means defining a second chamber disposed between said passage andsaid first chamber for receiving a supply of gas relatively inert tosaid volatile metallic element and includinga sheath opening forproviding a sheath stream of said inert gas between said jet stream andsaid stream of combustible gas sufficient when the torch is ignited toprevent residue from being formed on said nozzle aperture duringoperation.

2. The torch of claim 1 wherein the area of said sheath opening issubstantially greater than the area of said nozzle aperture.

3. The torch of claim 1 wherein said volatile metallic element comprisessilicon tetrachloride and said combustible gas comprises a mixture ofoxygen and hydrogen.

4. The torch of claim 1 wherein said nozzle aperture is circular, saidsheath opening is an annular opening concentrically disposed about saidnozzle aperture, and said nozzle openings are circular and are disposedsymmetrically about said annular opening.

5. The torch of claim 4 and further comprising eight nozzle openingsdisposed in a circle about said annular opening, each of said nozzleopenings having generally the same area as said nozzle aperture.

6. The torch of claim 1 and further comprising a chamber enclosing saidfirst chamber and having inlet and outlet means for circulation of acooling fluid therethrough.

7. A torch for forming a silica article directly upon a mandrel byhydrolysis of silicon tetrachloride entrained in a carrier gascomprising:

a. a torch housing having a central passage therethrough with an inletat one end for receiving gaseous silicon tetrachloride entrained in acarrier gas and having a nozzle aperture at the other end to provide anoutput jet stream of gaseous silicon tetrachloride entrained in acarrier gas;

b. a sheath chamber defined in said torch housing about said centralpassage including an inlet for receiving a relatively inert gas withrespect to silicon tetrachloride and having an annular openingconcentrically disposed about said nozzle aperture to provide a circularstream of said inert gas around said jet stream; and

c. a mixing chamber disposed about said sheath chamber including inletsfor hydrogen and oxygen containing gases and a plurality of outletopenings disposed symmetrically adjacent said annular opening to providestreams of gaseous combustible mixture around said circular stream ofsaid inert gas, whereby the flame resulting from ignition of said torchmay be kept away from said nozzle.

8. The torch of claim 7 and further comprising a cooling chamberdisposed about said mixing chamber having means for the circulation ofcooling fluid therethrough.

9. The torch of claim 7 wherein the area of said annular opening issubstantially greater than the area of said nozzle aperture. 7

10. A torch for direct formation of an oxide article on a mandrel bythey hydrolysis of a volatile metal chloride comprising:

a. a torch housing including inlet means for receiving a supply of thevapor of said volatile metal chloride;

gas is supplied through an annular opening surrounding a nozzleaperture, and each said stream of combustible gas is supplied from anaperture spaced radially outwardly from said annular opening.

12. The torch of claim 11 wherein said nozzle apertures and annularopenings are linearly disposed to one another, and said apertures forproviding combustible gas are linearly disposed parallel to said nozzleapertures and annular openings.

2. The torch of claim 1 wherein the area of said sheath opening issubstantially greater than the area of said nozzle aperture.
 3. Thetorch of claim 1 wherein said volatile metallic element comprisessilicon tetrachloride and said combustible gas comprises a mixture ofoxygen and hydrogen.
 4. The torch of claim 1 wherein said nozzleaperture is circular, said sheath opening is an annular openingconcentrically disposed about said nozzle aperture, and said nozzleopenings are circular and are disposed symmetrically about said annularopening.
 5. The torch of claim 4 and further comprising eight nozzleopenings disposed in a circle about said annular opening, each of saidnozzle openings having generally the same area as said nozzle aperture.6. The torch of claim 1 and further comprising a chamber enclosing saidfirst chamber and having inlet and outlet means for circulation of acooling fluid therethrough.
 7. A torch for forming a silica articledirectly upon a mandrel by hydrolysis of silicon tetrachloride entrainedin a carrier gas comprising: a. a torch housing having a central passagetherethrough with an inlet at one end for receiving gaseous silicontetrachloride entrained in a carrier gas and having a nozzle aperture atthe other end to provide an output jet stream of gaseous silicontetrachloride entrained in a carrier gas; b. a sheath chamber defined insaid torch housing about said central passage including an inlet forreceiving a relatively inert gas with respect to silicon tetrachlorideand having an annular opening concentrically disposed about said nozzleaperture to provide a circular stream of said inert gas around said jetstream; and c. a mixing chamber disposed about said sheath chamberincluding inlets for hydrogen and oxygen containing gases and aplurality of outlet openings disposed symmetrically adjacent saidannular opening to provide streams of gaseous combustible mixture aroundsaid circular stream of said inert gas, whereby the flame resulting fromignition of said torch may be kept away from said nozzle.
 8. The torchof claim 7 and further comprising a cooling chamber disposed about saidmixing chamber having means for the circulation of cooling fluidtherethrough.
 9. The torch of claim 7 wherein the area of said annularopening is substantially greater than the area of said nozzle aperture.10. A torch for direct formation of an oxide article on a mandrel bythey hydrolysis of a volatile metal chloride comprising: a. a torchhousing including inlet means for receiving a supply of the vapor ofsaid volatile metal chloride; b. a plurality of nozzle aperturescommunicating with said inlet means for providing a plurality of outputjet streams of said volatile metal chloride; c. means for supplying asheath of relatively inert gas around each of said jet streams; and d.means for supplying a stream of combustible gas adjacent each saidsheath of inert gas, whereby when said torch is ignited a plurality offlames will be provided for the direct formation of an oxide article onthe mandrel.
 11. The torch of claim 10 wherein each said sheath of inertgas is supplied through an annular opening surrounding a nozzleaperture, and each said stream of combustible gas is supplied from anaperture spaced radially outwardly from said annular opening.
 12. Thetorch of claim 11 wherein said nozzle apertures and annular openings arelinearly disposed to one another, and said apertures for providingcombustible gas are linearly disposed parallel to said nozzle aperturesand annular openings.